SC2440AEVB datasheet(18/28 Pages) SEMTECH

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Applications Information (Continued)

Figure 7(e)-(g). Bootstrap Configurations for the SC2440A (Continued)

voltage should not exceed its absolute maximum rating

of 42V. To reduce BST voltage stress when stepping down

from high V

IN (>20V) to low VOUT (<2.5V), a Zener diode can

be added in series with D

BST. This is shown in Figure 7(d).

The Zener voltage can be selected using (9):

(9)

Figures 7(e) and (f) show how to bootstrap the SC2440A

from a second independent power supply V

S. In Figure

7(g), the channel output is used as the bootstrap power

supply for channel 2. DC-DC regulators using this boot-

strap method are shown in Figure 6(a). If channel is out

of regulation, then channel 2 will be shut off by PGOOD.

Correct operation of channel 2 thus depends on the read-

iness of V

OUT. This may be a drawback.

SC2440A

(g)

SC2440A

BST1

GND

SW1

DBST1

CBST1

VOUT1 > 2.5V

VIN

DFW 1

SW2

BST2

DFW 2

VOUT2

CBST2

DBST2

PGOOD1

SS2

SC2440A

BST

GND

DBST

VOUT

CBST

VIN

VS > 2.5V

DFW

(e)

VS > VIN + 2.5V

SC2440A

BST

GND

DBST

VOUT

VIN

DFW

(f)

that using a Schottky bootstrapping diode when V

OUT >

2.5V produces no noticeable efficiency benefit.

If V

IN(MAX) + VOUT > 42V, then a Zener diode DZ can be used in

series with D

BST to lower the BST voltage [Figure 7(c)]. The

following inequality gives a suitable range for the Zener

diode voltage V

(8)

The SC2440A can also be bootstrapped from the input

[Figure 7(b)]. This configuration is not as efficient as Figure

7(a). However this may be the only option if the output

voltage is less than 2.5V and there is no other available

supply with voltage higher than 2.5V. Voltage stress at the

BST pin can be somewhat higher than 2V

IN. The BST pin

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